During cytokinesis, cleavage furrow invagination requires a protein-based contractile ring and vesicle-based addition of new membrane. Although membrane trafficking events play a fundamental role in cell division, how these events promote cytokinesis remains unclear. This proposal seeks to identify genes that promote cytokinesis by targeting vesicles to cleavage furrows. 3 distinct screens in living Drosophila embryos will be performed, including candidate mutants, deficiencies, and temperature-sensitive alleles. Previous studies have visualized membrane trafficking components in fixed embryos. Using GFP-fusion proteins that label vesicles, the proposed screens will monitor targeted vesicle delivery to furrows in living embryos. Primary screens will assay for mislocalization of vesicles and abnormal furrow ingression. Secondary screens will identify genes that directly link vesicle transport and cytokinesis; GFP-fusions that label various membrane trafficking components will be used to assay for defects within exocytic, endocytic, and recycling pathways. Outstanding issues in the field will be addressed, such as what are the sources of new membrane, mechanisms of vesicle transport, and if vesicles deliver proteins essential for cytokinesis.